Emulsion and Microemulsion Systems to Improve Functional Edible Oils Enriched with Walnut and Pistachio Phenolic Extracts

Abstract

The purpose of this research was to improve the properties of functional edible oils with potential health promoting effects, enriched with phenolic-rich extracts obtained from pistachio and walnut (5.1 and 27.4% phenolic contents respectively), by means of emulsion and micro emulsion systems. Stable water-in-oil (W/O) emulsions were obtained employing polyglycerol polyrhizinoleate (PGPR) as emulsifier (0.5, 2% H₂O in oil), despite having a whitish and opaque appearance; transparent and stable microemulsions were prepared using proper proportion (e.g., 97:3) between the oily phase and the mixture of aqueous phase and emulsifiers (3:2 lecithin-distilled monoglycerides (DMG). Total polar phenolics contents ranging between 257 and 835 mg/kg were obtained in the novel functional edible oils' formulations, reaching higher content using walnut as compared to pistachio extracts. Antioxidant capacity determined by the 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl (DPPH) method increased approx. 7.5 and 1.5 times using walnut and pistachio extracts respectively. An emulsion using gallic acid and a microemulsion employing hydroxytyrosol, two well-known antioxidants, were also studied to compare antioxidant capacity of the proposed enriched oils. Furthermore, the oxidative stability of these products-very relevant to establish their commercial value-was measured under accelerated testing conditions employing the Rancimat equipment (100 °C) and performing an oven test (at 40 °C for walnut oils and 60 °C for pistachio and refined olive oils). Rancimat oxidative stability greatly increased and better results were obtained with walnut (2-3 times higher) as compared to pistachio extract enriched oils (1.5-2 times higher). On the contrary, under the oven test conditions, both the initial oxidation rate constant and the time required to reach a value of peroxide value equal to 15 (upper commercial category limit), indicated that under these assay conditions the protection against oxidation is higher using pistachio extract (2-4 times higher) than walnut's (1.5-2 times higher). Stable emulsions and transparent microemulsions phenolic-rich nut oils (250-800 mg/kg) were therefore developed, possessing a higher oxidative stability (1.5-4 times) and DPPH antioxidant capacity (1.5-7.5 times).

Keywords: antioxidant capacity; functional oils; phenolic extracts; pistachio; walnut.

Figure 1

Properties of formulated microemulsions with different oil, water and surfactant ternary proportions. white dots represent transparent microemulsions, while grey once correspond to opaque-looking ones.

Figure 2

Evolution of the oxidation process of oil formulations under oven test * conditions. Oven Test: 40 °C in VWO; 60 °C in VPO and ROO. VWO, virgin walnut oil; VPO, virgin pistachio oil; ROO, refined olive oil. Wex, walnut phenolic extract; Pex, pistachio phenolic extract; gal, gallic acid.